Most sensory systems have a mechanism for comparison that, through exaggeration of perceived stimulus difference, serves to facilitate accurate detection of, and responding to, a range of stimuli. Such an effect can be demonstrated in the gustatory system by varying the temporal presentation of different concentrations of a normally preferred stimulus. Specifically, when presented in separate sessions, rats will demonstrate a concentration effect by licking more for a high than for a low concentration of sucrose. When presentation of the two concentrations is alternated within the same session, however, the concentration effect is exaggerated 2-3 times. In behavioral studies this exaggeration in referred to as a Simultaneous Contrast Effect. That is, licking for the high concentration is enhanced (positive contrast) and licking for the low concentration is suppressed (negative contrast) when rats are given the opportunity to compare the two closely in time. This effect can be likened to an"edge effect" in the visual system. Despite the likely contribution of such comparison effects to detecting, coding, and responding to gustatory stimuli, the behavioral and electrophysiological assessment of taste processing has been limited to investigation during conditions designed to prevent stimulus comparison. For example, behavioral responsiveness to stimulus quality and intensity typically is evaluated using random stimulus delivery in an effort to reduce the potential effects of prior experience on ingestive behavior. During electrophysiological experiments, a similar attempt is made to eliminate order effects by presenting the stimuli in a rinse-stimulus-rinse sequence. The present proposal seeks to establish the neural basis for this comparison process. Preliminary data indicate that an intact brainstem is sufficient to mediate the behavioral expression of Simultaneous Contrast Effects. Other experiments, however, discount adaptation of the peripheral receptors as an explanation of the phenomenon and rule out involvement of the pontine parabrachial nuclei. This leaves the nucleus of the solitary tract (NST) as a potential site for the neural mechanisms that produce athe effect. In fact, our preliminary data demonstrate exaggerated responses to alternating concentrations of sucrose, NaCl, or citric acid in NST units. This effect occurs for sucrose even when the two concentrations are applied to separate receptive fields, further discounting an adaptation interpretation of the phenomenon. The focus of this proposal is to: (1) select the sucrose concentration pair associated with the largest contrast effects in behavior; (2) examine the effect of comparison and non-comparison conditions (using the Simultaneous Contrast procedure) on gustatory coding in NST units; (3) determine the parametric constraints on contrast effects both behaviorally and electrophysiologically; and (4), extend pilot data that rule out peripheral adaptation as the sole mechanism for Simultaneous Contrast.